Oxygen partial pressure sensor

ABSTRACT

A pressure programmed, piezoelectric sensor for the partial pressure of oxygen in a helium-oxygen breathing gas mixture is described, together with associated electronic circuitry for use in a diving apparatus. The sensor utilizes acoustic signal phase shift to generate electrical signals by which the apparatus maintains oxygen partial pressure within the required limits for breathing at various water depths.

[73] Assignee:

[22] Filed:

Appl. No.:

OXYGEN PARTIAL PRESSURE SENSOR Inventors: Carl G. Ringwall, Scotia, N.Y.;

William L. Keltz, Westchester, Pa.

The United States of America as represented by the Secretary of the Navy, Washington, DC.

Mar. 9, 1973 References Cited UNITED STATES PATENTS Kanwisher 128/142 Burk et a1. Holmes 128/142 US. Cl. 73/24, 128/142 Int. Cl A62b 7/02 Field of Search 73/23, 24; 128/140 R, 142

[451 Apr. 23, 1974 2,283,750 5/1942 Mikelson 73/24 3,722,510 3/1973 Parker 128/142 2,998,009 8/1961 Holm et a1. 73/23 x Primary Examiner-Richard C. Queisser Assistant Examiner-Stephen A. Kreitman Attorney, Agent, or Firm-Richard S. Sciascia; Don D. Doty; Harvey A. David 5 7] ABSTRACT 4 Claims, 5 Drawing Figures ems 5o 10 OSCILLATOR 4/ 38 SOURCE 4o PHASE SHIFTER /52 3s 32 PHASE 42 30 4e DISCRIMINATOR 7/ r1 r1; '1 48 PARTIAL 44 7/ PRESSURE AMPLIFIER SENSOR SOLENOID 7 OXYGEN SCRUBBER 2 VALVE SUPPLY BREATHING BAG,ETC

?ATENTEBISPR 2'3 F914 SHEET 1 [1F 3 OSCILLATOR W34 BIAS 5O IO 38 SOURCE I PHASE 52 SHIFTER 36 II 32 PHASE 42 30 4e DISCRIMINATOR *I H 2 1 48 PARTIAL 44 7/ PRESSURE D AMPLIFIER SENSOR 20 22 SOLENOID "I OXYGEN SCRUBBER '5 VALVE SUPPLY II I42, 4A6 4/ BREATHING FlG.I

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9ATENTEDAPR23 1914 SHEET 2 OF 3 1 OXYGEN PARTIAL PRESSURE SENSOR STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

FIELD OF THE INVENTION This invention relates to improved apparatus for analyzing a gas by its acoustic transmission properties, and more particularly to apparatus useful in determining the partial pressure of oxygen in a helium-oxygen breathing gas mixture, for example, in a deep diving apparatus.

Helium-oxygen mixtures are provided as the breathing gas for deep diving purposes to eliminate the narcotic effects and the possibility of oxygen poisoning attendant the use of compressed air breathed at substantial depths, and also to reduce the time required to decompress a diver without inducing caisson disease or bends." The oxygen content of a helium-oxygen mixture, in order to satisfy the physiological needs of the diver for oxygen and at the same time to avoid likelihood of oxygen poisoning, must be such that the partial pressure of the oxygen is within a predetermined range which is independent of the absolute pressure at which it is to be breathed. Because the oxygen partial pressure requirements of a diver remain unchanged with changes in pressure, it becomes necessary to adjust the ratio of oxygen to helium in the breathed mixture for the different pressures experienced at different water depths.

DISCUSSION OF THE PRIOR ART In existing systems, wherein the breathing gas is wholly or partially recirculated, oxygen which is removed during the breathing process is replaced by oxygen from a suitable supply, such as a pressurized tank. The rate at which oxygen is replaced to enrich the helium-oxygen mixture is controlled to maintain an oxygen partial pressure which is within the desired limits. The control has been effected in the past by electrochemical sensors which sense the amount of oxygen present in the breathing gas mixture, and provide electrical signals that are used to control a solenoid valve on the oxygen supply as necessary to maintain the desired mixture. The available electrochemical sensors are not wholly satisfactory in that their reliability, accuracy, and useful life are limited. With respect to accuracy, it should be noted that the response of electrochemical sensors to changes in oxygen content is not as rapid as is desired considering the short time a diver may have to correct or remedy his situation when a malfunction occurs. Accordingly, it has been the practice in certain divers helium-oxygen breathing systems to utilize redundancy of sensors as a safeguard against malfunction and attendant incorrect mixture. Naturally, it would be desirable to have, as part of a helium-oxygen breathing system, a more reliable, durable, fast acting, and accurate oxygen partial pressure determining means. Preferably one which utilizes physical rather than chemical properties to generate a control signal for the oxygen replenishing valve.

lt has been known for some time that the acoustic transmission properties of a gas or gas mixture are related to the temperature, pressure, and makeup of the gas or mixture. Accordingly, various analytical test devices have been made which use physical properties, such as the velocity of sound transmission through sample and reference quantities of gases as a measure of factors, such as percentage of a certain gas in a mixture, to provide indicating or controlling electrical signals. These analytical devices, typical examples of which are described in U.S. Pat. Nos. 2,952,153 and 3,557,605, are laboratory instruments that have no provisions for operation either in an aquatic medium or under varying ambient pressure conditions such as those to which a diver and his breathing equipment are subjected at depths ranging to a thousand feet or more.

SUMMARY OF THE INVENTION The present invention aims to overcome some or all of the aforementioned disadvantages and shortcomings of the prior art through the provision of a novel oxygen partial pressure sensing apparatus and system which utilizes the acoustic transmission properties of heliumoxygen mixtures as the determining factor, and which is eminently suited to use in controlling the amount of oxygen in a divers breathing gas mixture at various depths.

With the above in mind, it is a principal object of the invention to provide an improved oxygen partial pressure sensing apparatus which utilizes only physical parameters, rather than chemical, to generate an electrical signal representative of the state of oxygen content in a helium-oxygen breathing gas mixture.

Another important object of this invention is the provision of an oxygen content responsive control system for a divers breathing gas supply, which serves to maintain the oxygen content within a predetermined range of values irrespective of operating depth and pressures at which the system is operated within a predetermined range of depths and pressures.

Another object is the provision of such a system including a piezoelectric element for transmitting a sound signal of predetermined frequency, through the gas mixture to be analyzed, to a receiving piezoelectric element, and circuitry for generating an electrical voltage signal the voltage of which is representative of the amount of oxygen in the mixture.

Still another object is the provision of a system of the foregoing character which does not require the use of a reference gas mixture for comparison during use.

Yet another object is the provision of an accurate, reliable, substantially instantaneous and continuous gas mixture quality sensing device which automatically compensates for temperature and pressure condition changes.

As another object it is an aim of the invention to accomplish the foregoing through the use of novel pressure responsive mechanisms to automatically vary the distance between the transmitting and receiving piezoelectric elements, and through the agency of thermally responsive means in associated electronic circuitry.

Theinvention may be further said to reside in certain novel combinations, constructions, and arrangements of parts by which the foregoing objects and advantages are achieved.

Other objects and many of the attendant advantages will be readily appreciated as the subject invention becomes better understood by reference to the following detailed description, when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic illustration, in block form, of a helium-oxygen breathing system utilizing oxygen partial pressure sensing apparatus embodying the invention;

FIG. 2 is a longitudinal sectional view of a portion of the apparatus of FIG. 1;

FIG. 3 is a transverse sectional view taken substantially along line 33 of FIG. 2;

FIG. 4 is a fragmentary elevational view taken along line'44 of FIG. 2; and FIG. 5 is a schematic illustration of electronic circuitry of the system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, the invention will be described with reference to use in a helium-oxygen underwater breathing apparatus for divers, generally indicated at 10. System comprises closed circuit breathing apparatus including a breathing bag and other paraphernalia, represented by block 12, connected as shown by lines 14 and 16 for circulation of exhaled heliumoxygen mixture through a scrubber l8. Scrubber 18 is usually in the form of a canister, and serves to remove exhaled carbon dioxide from the breathing gas mixture. The scrubbed helium-oxygen mixture is then rebreathed by the diver and then returned to the scrubber for carbon dioxide removal. Because this recirculation of the helium-oxygen breathing mixture results in a depletion of the oxygen content thereof, it is necessary to replenish the oxygen at a rate which will maintain the partial pressure of oxygen in the mixture within predetermined minimum and maximum limits for the ambient pressures at which the system 10 is being used. To this end, there is provided a supply of oxygen 20, that is connected as shown by line 22, a solenoid valve 24, and a line 26 to scrubber l8.

' Operation of solenoid valve 24 to admit oxygen from supply 22 to scrubber 18 is, according to the present invention, under the control of a novel partial pressure sensor 30 which is mounted on scrubber l8 and operates in conjunction with electronic circuitry to develop electrical control signals for the solenoid valve. Sensor 30, which will be described in detail with reference to F IGS. 2 and 3, utilize the fact that the acoustic transmission properties of a gas mixture are predictably effected by changes in the quantity of the constituents, changes in temperature, and changes in pressure thereof.

Sensor 30 is provided with alternating current electrical input signals, represented by flow line 32, from a temperature responsive oscillator 34. The frequency of signals on line 32 vary in accordance with the ambient temperature at which the system 10 is operating in a manner, and for a purpose, which will be made apparent as the description proceeds.

The electrical signal output of oscillator 34 is applied also, as shown by line 36, to a 45 phase shifter 38. The output of phase shifter 38, represented by line 40, is at the same frequecy as the output of oscillator 34, but lags that output by 45. The phase shifted output of phase shifter 38 is applied as a first input to a phase discriminator 42.

Partial pressure sensor 30 provides an alternating current electrical output signal represented by line 44, which output signal bears a phase relation to the input signal on line 32 of 360 whenever the oxygen partial pressure in the helium-oxygen mix in scrubber 18 is at the desired value for the ambient temperature and pressure conditions on the system'lfl. The output of partial pressure sensor 30 is amplified by an amplifier 46 and applied as shown via line 48 as a second input to phase discriminator 42. Amplifier 46 introduces a 45 phase shift in the opposite direction to that introduced by phase shifter 38. The operation of phase discriminator 42, which is provided a bias voltage from source 50 via line 52, is such as to provide a predetermined control signal via line 54 to solenoid valve 24, which control signal varies in accordance with the amount that the output signal on line 44 from pressure sensor 30 leads or lags the input signal via line 32 in phase. The electrical control signal output of phase discriminator 42 causes solenoid valve 24 to control the flow of oxygen from supply 20 to scrubber 18 in such a manner as to seek an oxygen partial pressure condition in scrubber 18 which will result in the mentioned 360 phase shift between input and output signals of the sensor 30. Accordingly, the system 10 serves to seek an average value of oxygen partial pressure within the desired predetermined range.

Referring now to FIGS. 2 and 3, partial pressure sensor 30 comprises a generally cylindrical housing body 60 formed of a suitable rigid, corrosion resistant material such as stainless steel. Housing body 60 comprises cylindrical side wall 60a extending from a circular end wall 60b, which body defines an interior cavity 62 which is closed on the end opposite wall 601) by an end wall in the form ofa closure member 64. Closure member 64 has a circular portion 64a received within the circular wall portion 60a, and a generally square flange portion 64b which is a congruent with a square flange portion 600 extending outwardly from wall portion 600. Closure member 64 has its flange portion 64b secured to flange portion 600 by machine screws 66. An'O-ring 68 is provided in a groove in closure member portion 64a and serves to effect a watertight seal between closure member 64 and body 60.

Extending outwardly from wall portion 60b is an externally threaded annular boss 70 which extends through an opening in a wall 72 of scrubber 18. Boss 70 defines an opening 74 into the interior 62 of housing body 60 and has received thereon a nut 76 in clamping engagement with the wall 72. An O-ring 80 is received in an annular groove 82 surrounding boss 70, and serves to effect a watertight seal between scrubber 18 and housing body 60.

Closure member 64 is provided with a central recess 86, in which is disposed a pressure responsive bellows 90. One end of bellows acts against cover member 64 while the inner end 90b thereof carries an axially movable shaft 92. Shaft 92 is characterized by a head portion 92a which is secured to end 901) of bellows 90 as by soldering, a reduced portion 92b having a radially extending key 92c, and a still further reduced portion 92d that is threaded to receive a nut 94.

Shaft 92b extends through the center of a support spring that comprises a washer like central portion 100a which lies against flanged head 92a, and is connected by three serpentine leg portions 1001) to an outer ring portion 100a. Outer ring portion 100c is clamped between cover member 64 and a ring 102 that bears against an annular shoulder 104 formed in cylindrical wall portion 60a of body 60. The serpentine leg portions are best illustrated in FIG. 3.

Support spring 100 serves to prevent lateral displacement of shaft 92, while permitting that shaft to make substantial axial excursions under the influence of bellows 90. Disposed on shaft 92, adjacent support spring 100, is a disc 106 which is adapted to cooperate with a plurality of stop members, in the form of screws 108, to limit movement of shaft 92 in the direction of bellows 90. Screws 108 are adjustable, in the threaded bores 110 of cover member 64 in which they are received, in order to selectively determine the limit of movement of shaft 92.

Next to disc 106, on shaft 92, is a spacer washer 108, which is followed by a plurality of three-legged leaf springs 110, 112, 114, and 116. These springs, especially their three-legged configuration, are best illustrated in FIG. 3. The springs 110 and 112 are disposed in registration with one another to act as a compound spring, whereas springs 114 and 116 are angularly offset with respect to one another and with respect to springs 110 and 112 so as to act independently. The legs of springs 110 and 112 are adapted to engage the ends of adjustable screws 120 at a predetermined position of axial movement of shaft 92 toward bellows 90, and thereafter to introduce a predetermined resistance to further movement of shaft 92 in accordance with the spring rate of the legs thereof, and for a purpose which will be made apparent during the discussion of the mode of operation of the invention.

The three legs of spring 1 14 are adapted to cooperate in a similar manner with screws 122, whereas the three legs of spring 116 are adapted to cooperate with adjustable screws 124. The springs 110, 112, 114, and 116 are maintained in their respective positions with respect to shaft 92 by engagement with the key 92c of that shaft.

Following spring 116 on shaft 92 are a spacer washer 126 and a mounting arm of a transmitting crystal carrying member 130. The arm 130a, as well as the preceding elements on shaft 92, is secured by nut 94. Nut 94 is conveniently prevented from loosening by a retainer 132.

Crystal carrying member 130 comprises a cup shaped portion 130/!) which is centrally arranged in opening 74, and serves to carry a piezoelectric, acoustic signal transmitting crystal 136. It will be appreciated that axial movement of shaft 92 will be accompanied by corresponding translational movements of crystal 136 along a path normal to face 136a of the crystal.

Mounted in bridging relation to opening 74, in boss 70, is a receiving crystal mounting member 140, the ends of which are secured as by screws 142 to ledges 144 (FIG. 4). Crystal mounting member 140 comprises a centrally located cup shaped portion 140a having a central opening l40b located in alignment with crystal 136. A receiving piezoelectric crystal 148 is mounted in cup portion 140a and presents a surface 148a in spaced parallel relation to surface 136a of crystal 136. Crystal 136 is connected by suitable leads or wires 150 which are led through a standoff tube 152 to a waterproof electrical connector 154 for connection to oscillator 34. Crystal 148 is likewise connected by suitable leads or wires 156 to a waterproof connector 158 for connections to amplifier 46. It will be recognized that helium-oxygen gas mixture within scrubber 18 will extend through opening 74 to the interior 62 of the sensor 30, including the space 86 around bellows and the space defined between surfaces 136a and 148a of crystals 136 and 148, respectively. This gas mixture will be at a pressure corresponding substantially to the pressure of the surrounding water at the depth of use, and because of the movementof the gases in scrubber l8 and the extension of boss 70 into that scrubber canister, the gas mixture which is present between the crystals 136 and 148 will be representative of gas passing through the scrubber at any particular time. Accordingly, acoustic energy emanated from surface 136a of crystal 136 in response to'electrical signals provided by oscillator 34, will travel through helium-oxygen gas mixture to crystal 148 for reconversion to electrical signals. The phase relationship between the electrical signals driving crystal 136 and the electrical signals produced by crystal 148, will be a function of the distance between surfaces 136a, 148a, the temperature of the gas therebetween, the pressure of the gas, and the percentage of oxygen whose partial pressure contributes to that pressure.

Referring now to FIG. 5, there will be described an electronic circuit which is a composite of the electronic elements indicated in the block diagram of FIG. 1.

Thus, the circuit of FIG. 5 has a plurality of sections indicated generally by reference 34, 38, 42, and 46 corresponding to the functional elements of FIG. 1.

Temperature responsive oscillator 34, in the exemplary circuit being described, comprises a single NPN transistor and a LC tank circuit comprising an inductor 172 in parallel with capacitors 174 and 176. This oscillator is a more or less conventional Colpitts configuration wherein negative feedback from the tanks circuit is provided by resistor 178 to the emitter of transistor 170. Power for the circuit is derived from a suitable source of positive 12 volt direct current represented by terminal 180, and negative 12 volt direct current represented byterminal 190, and which sources are connected via lines 182, 184, and resistors 186, 188 respectively to line 192 and 194. Lines 192 and 194 are connected through capacitors 196 and 198 to ground, and through resistors 200 and 202 to the base of transistor 170. Resistors 200 and 202 from a voltage divider to provide appropriate bias to the base of that transistor. A capacitor 204 is connected between the base of transistor 170 and the negative line 194, and a load resistor 208 is connected between the emitter of that transistor and the negative line 194 to complete the oscillator circuit. The output of the oscillator, the frequency of which is determined by the tank circuit elements 172, 174, and 176, is taken from the collector of transistor 170 and applied via line 32 to the transmitting crystal 136 within the oxygen partial pressure sensor 30.

The oscillator provides temperature compensation for the oxygen sensor 30 and the system in which it is employed. Temperature compensation requires that the oscillator frequency vary with temperature in the same manner as the acoustic velocity in the heliumoxygen mixture varies with temperature. This requirement is satisfied if the LC product of the oscillator tank circuit is made inversely proportional to absolute temperature. There are a wide variety of both inductor core materials and capacitors which have negative temperature coefficients in the required range (0.2 percent per Fahrenheit). 1n the exemplary embodiment being described the inductor 172 contributes about equally with the combined effect of capacitors 174 and 176 to provide the necessary compensation.

A portion of the output of the oscillator 34 is applied via a resistor 210 to the base of the transistor 212 that serves to drive the phase shift network 38. The phase shift network, which-introduces a 45 phase lag to the oscillator frequency output, comprises an inductor 214, and a capacitor 216 and resistor 218. The emitter of transistor 212 is connected through a resistor 220 and through the phase shift network elements 214, 216, and 218 to ground. The collector-emitter circuit of transistor 212 is completed by a line 222 to the positive current source via lines 192 and resistor 186. Appropriate biasing conditions are provided for the base of transistor 212 by voltage divider resistors 224 and 226.

The phase lag output of phase shift network 38 is applied via line 40 to the base of a transistor 230 forming part of the phase discriminator 42, and which transistor has its collector connected to line 192 and its emitter connected through resistor 232 to the primary winding 234 of the transformer 236. The other input to phase discriminator 42 is derived from crystal 148, which is connected as shown by line 44 to the amplifier 46. Amplifier 46 comprises two transistor stages represented by transistors 240 and 242. The input on line 44 is connected to the base of transistor 240, which base is provided with appropriate biasing by voltage divider resistors 244 and 246. The collector-emitter circuit of transistor 240 is connectedbetween the positive and the negative voltage sources as shown by resistors 248, 250, 252, and 254. An output taken from the first amplifier stage transistor 240 is coupled via capacitor 256 to the base of the second stage transistor 242, which base is appropriately biased by voltage divider resistors 258 and 259. The transistor is further connected between the positive and the negative voltage sources by resistors 260, 262, 264, and 266. The amplifier stages are connected, as shown, through capacitors 268, 270, 272, and 274 to ground. An emitter bypass capacitor 276 is connected in series with a resistor 278 across resistor 252, while a bypass capacitor 280 is connected in series with a resistor 282 across resistor 264 serving the emitter of transistor 242. v

The output of the second state transistor 242 is taken from the collector thereof and applied via coupling capacitor 286 to the junction 288 between diodes 290 and 292 of the phase discriminator 42. The amplifier circuit 46 introduces a 45,phase lead to the output signal of receiving crystal 148.

Secondary winding 294 of phase discriminator 42 has one end connected through resistor 300 and diode 292 junction 288 while the other end of that winding is connected through a resistor 302 and diode 292 to junction 288. The center tap of winding 294 is connected through an RC circuit comprising resistor 304 and capacitors 306 in parallel therewith, to line 308 connected to the negative voltage source and through a resistor 310 to junction 288. A resistor 312 is provided between line 308 and ground, and serves to establish the desired bias voltage applied to the discriminator and represented by block 50 of P16. 1.

The phase discriminator is characterized by a null output on line 54 thereof, from the center-tapped secondary winding 294 of transformer 236, whenever a 90 phase difference exists between the input to pri- The primary of transformer 236 is driven, as described, by the crystal'driving oscillator 34 and'establishes a reference voltage for the discriminator. The voltage on the secondary of transformer 236 causes the diodes 290, 292 to conduct on one-half cycle of the reference voltage. During the conducting portion of the cycle, the input to the discriminator is clamped to the center tap of the transformer. The signal appearing on the center tap is then filtered to produce an average output. If the input signal is phase shifted relative to the reference voltage, an equal negative and positive voltage appears at the center tap and the average output is zero. If the input at junction 288 is in phase with the reference applied to the primary, a positive half wave rectified sine wave will appear on the center tap. 1f the input is out of phase, the rectified sine wave will be of negative polarity. The polarity of the signal output on line 54 is used as an indicator of whether the phase of the signal input from crystal 148 is greater or less than 360 with respect to the output of crystal 136, and the magnitude of the signal on line 54 is represen tative of the phase shift error. The output on line 54 is used to control the solenoid valve 24- so that the system of FIG. 1 functions to maintain the discriminator in a nulled condition.

1n the embodiment being described, the values of components used in the circuitry of FIG 5 are as follows:

Resistor 178 1.2 K G 186 1 188 130 Q 200 33 K .(1 202 10 K (1 208, 680 K!) 210 47.5 K!) 218 15 KO 220 1.6 KG 224 47.5 KB 226 47.5 KO 232 750 .0 244 24.9 KS1 246 13 KO 248 130 KG 250 4.6 K9 252 4.6 KG 254 130 Q 258 24.9 KO 259 13K 260 130 O 262 4.6 KG 264 4.6 KO 266 130 Q 282 300 Q 300 39 KG 302 39 KO 304 I21 K!) 310 121 KO 312 130 1(1) Capacitor 174 0.005 pf 176 0.0075 #1 196 3 pf 198 3 pl 204 3 at 216 0.002 #f 256 0.01 if 268 3 cf 270 3 pl 272 3 pif 274 3 put 286 0.01 ptf Transistor Diode 290 lNH306 292 lNl-l306 lnductor MODE OF OPERATION When a dive utilizing the deep diving system embodying the invention is to be made, the system is charged with a helium oxygen mixture which will be suitable in oxygen partial pressure for work near the surface. At or near the surface, bellows 90 will have established a spacing for crystals 136, 148, which results in an electrical control signal output from discriminator 42 that will cause solenoid valve 24 to admit replenishing oxygen as necessary to maintain the oxygen partial pressure within a suitable range for the near surface pressure condition. Also, temperature responsive oscillator 33 will have established a frequency for its output that is determined by the ambient water temperature (and that of the system and breathing gas).

As the diver descends to greater depths, bellows 90 will be compressed to a degree determinedby the water pressure which is reflected in the pressure of the helium-oxygen mixture which must closely follow the water pressure to permit breathing by the diver. The bellows, of course, has a predetermined spring rate opposing compression due to its construction from a suitable metal, and due to its gas fill. Compression of bellows 90 causes transmitting crystal 136 to move away from the receiving crystal 148, thereby introducing a phase difference in input and output signals of sensor 30. This phase difference is utilized by discriminator 42 to alter the control signals to solenoid valve 24 in a direction and amount that will cause the oxygen partial pressure to be changed so that the acoustic properties of the gas mixture will reestablish the 360 phase relation between the input and output signals of sensor 30.

Because the required changes in the ratio of O to H partial pressures as depths increase are not linear, the effective spring rate of the bellows 90 must be changed. This is the purpose and function of the plurality of three-legged spring elements 110, 112, 114, and 116 that cooperate with stop screws 120, 122, and 124. By having these springs come successively into operation, the rate at which the spacing between the transmitting and receiving crystals changes with depth is varied. Of course, more or fewer springs could be used depending upon total range of depths the'system is to be used with, and depending upon how closely it is desirable to approximate the optimum oxygen to helium partial pressure ratio to depth function.

As the diver ascends the system likewise acts to alter the oxygen partial pressure in accordance with depth so that a substantially optimum oxygen content is maintained.

Obviously, other embodiments and modifications of within the scope of the appended claims.

What is claimed is:

l. A sensor device for use in determining oxygen partial pressure in mixed gas breathing apparatus, said device comprising:

a housing defining a central cavity, said housing comprising side walls and oppositely disposed end walls;

an opening defined in one of said end walls for communication of mixed gas between said cavity and the exterior of said housing;

an expansible bellows disposed in said housing having one end mounted on the other of said end walls;

a shaft mounted on the other end of said bellows for axial movement by said bellows in response to changes in total pressure of said mixed gas;

a plurality of multi-legged spring elements mounted on said shaft for movement therewithi, said elements each having a plurality of radially extending legs;

a plurality of adjustable stop members extending from said other end wall, each of said stop members being engageable by one of said legs of said spring elements at predetermined axial positions of said shaft;

a first piezoelectric transducer element mounted on.

said housing;

a second piezoelectric transducer element mounted on said shaft in spaced relation to said first piezoelectric transducer element and movable with said axial movement of said shaft toward and away from said first piezoelectric transducer element;

one of said piezoelectric transducer elements being responsive to first electrical signals having a characteristic frequency to transmit acoustic energy into said gas mix in a direction parallel to said axial movement of said shaft;

the other of said piezoelectric transducer elements being adapted to receive said acoustic energy and to produce second electrical signals of said characteristic frequency and having a phase relation to said first electrical signals that is a function of the positioning of said shaft and said second piezoelectric transducer element by said bellows and of said oxygen partial pressure in said mix; and

said positioning of said shaft and said second piezoelectric transducer element being a function of said total pressure and of the spring rates of said multi-legged spring elements, whereby differences from a predetermined phase relation may be taken as a measure of departure of oxygen partial pressure from the desired partial pressure for any existing total pressure within a predetermined range of total pressures.

2. A sensor device as defined in claim 1, and further characterized by:

support spring means, connected between said housing and said shaft, for confining said shaft to axial movements.

first, second, and third spring elements each having a central ring portion carried on said shaft and having three radially extending legs disposed equiangularly with respect to adjacent ones thereof;

said first, second, and third spring elements being disposed with respect to one another on said shaft such that the legs of each are angularly offset with respect to the legs of the others, whereby all of the legs have their ends free of interference from the others. 

1. A sensor device for use in determining oxYgen partial pressure in mixed gas breathing apparatus, said device comprising: a housing defining a central cavity, said housing comprising side walls and oppositely disposed end walls; an opening defined in one of said end walls for communication of mixed gas between said cavity and the exterior of said housing; an expansible bellows disposed in said housing having one end mounted on the other of said end walls; a shaft mounted on the other end of said bellows for axial movement by said bellows in response to changes in total pressure of said mixed gas; a plurality of multi-legged spring elements mounted on said shaft for movement therewithi, said elements each having a plurality of radially extending legs; a plurality of adjustable stop members extending from said other end wall, each of said stop members being engageable by one of said legs of said spring elements at predetermined axial positions of said shaft; a first piezoelectric transducer element mounted on said housing; a second piezoelectric transducer element mounted on said shaft in spaced relation to said first piezoelectric transducer element and movable with said axial movement of said shaft toward and away from said first piezoelectric transducer element; one of said piezoelectric transducer elements being responsive to first electrical signals having a characteristic frequency to transmit acoustic energy into said gas mix in a direction parallel to said axial movement of said shaft; the other of said piezoelectric transducer elements being adapted to receive said acoustic energy and to produce second electrical signals of said characteristic frequency and having a phase relation to said first electrical signals that is a function of the positioning of said shaft and said second piezoelectric transducer element by said bellows and of said oxygen partial pressure in said mix; and said positioning of said shaft and said second piezoelectric transducer element being a function of said total pressure and of the spring rates of said multi-legged spring elements, whereby differences from a predetermined phase relation may be taken as a measure of departure of oxygen partial pressure from the desired partial pressure for any existing total pressure within a predetermined range of total pressures.
 2. A sensor device as defined in claim 1, and further characterized by: support spring means, connected between said housing and said shaft, for confining said shaft to axial movements.
 3. A sensor device as defined in claim 2, and wherein said support spring means comprises: an outer ring portion of a first diameter engaging said housing; an inner ring portion of a second, smaller diameter engaging said shaft; and a plurality of resiliently flexible, serpentine leg portions interconnecting said outer and inner ring portions.
 4. A sensor device as defined in claim 3, and wherein said spring elements comprise: first, second, and third spring elements each having a central ring portion carried on said shaft and having three radially extending legs disposed equiangularly with respect to adjacent ones thereof; said first, second, and third spring elements being disposed with respect to one another on said shaft such that the legs of each are angularly offset with respect to the legs of the others, whereby all of the legs have their ends free of interference from the others. 